The proposed work is on basic physiology and pharmacology of contrast agents. We have previously studied 1) biliary and urinary excretion of cholecystographic agents; 2) the renal physiological response to intravenous pyelography; 3) the nature of acute diuresis; and 4) the osmotic activity of cholecystographic agents in bile. Our present work, and that for the near future, involves the analysis of binding of diagnostic agents to multiple sites on human serum albumin. For this purpose, techniques have been developed which involve competitive binding with a computerized solution for the binding affinities at separate sites. These indicate that some (not all) agents induce conformational changes which influence the binding of other compounds. This work will be extended to additional compounds. In addition, as an outgrowth of the studies on acute diuresis, the phenomenon of renal delay time will be studied for more compounds, particularly those which undergo active tubular secretion. BIBLIOGRAPHIC REFERENCES: Electrolyte excretion and free-water production during onset of acute diuresis. Mudge, G.H., Cooke, W.J. and Berndt, W.O. Am. J. Physiol. 228: 1304-1312, 1975. Effect of choleretics on biliary and urinary excretion of cholecystographic agents. Cooke, W.J. and Mudge, G>H. Invest. Radiol. 10: 189-196, 1975.